Like
mammals, the young of some birds are fed on special
secretions from a parent. Unlike mammals, however, both
sexes produce it. The best known of these secretions is the
"crop milk" that pigeons feed to squabs. The milk is
produced by a sloughing of fluid-filled cells from the
lining of the crop, a thin-walled, sac-like food-storage
chamber that projects outward from the bottom of the
esophagus. Crops are presumably a device for permitting
birds to gather and store food rapidly, minimizing the time
that they are exposed to predators. Crops tend to be
especially well developed in pigeons and game
birds.

Crop milk is extremely
nutritious. In one study, domestic chicks given feed
containing pigeon crop milk were 16 percent heavier at the
end of the experiment than chicks that did not receive the
supplement. The pigeon milk, which contains more protein and
fat than does cow or human milk, is the exclusive food of
the nestlings for several days after hatching, and both
adults feed it to the squabs for more than two weeks. The
young pigeons are not fed insects as are the chicks of many
seed-eating birds; instead, the crop milk provides the
critical ration of protein.

The milk of Greater
Flamingos contains much more fat and much less protein than
does pigeon milk, and its production is not localized in a
crop, but involves glands lining the entire upper digestive
tract. Interestingly, the milk contains an abundance of red
and white blood cells, which can be seen under the
microscope migrating like amoebas through the surface of the
glands. Young flamingos feed exclusively on this milk for
about two months, while the special filter-feeding apparatus
that they will later employ for foraging
develops.

Emperor Penguin chicks may
also be fed milk in some circumstances. Each male incubates
a single egg on his feet, covered with a fold of abdominal
skin, for two months of the Antarctic winter, fasting while
the female is out at sea feeding. If the female has not
returned with food by the time the chick hatches, the male
feeds it for a few days on milk secreted by the esophagus.
After its brief diet of milk, the chick will be fed by
regurgitation alternately by the male and female as they
travel one at a time to the sea to hunt.

Thus three very different
groups of birds have evolved the capacity to produce milk as
solutions to very different problems: the need for protein
and fat in the pigeons, which feed very little animal
material to the squabs; the need for liquid food consumption
during the development of the specialized feeding apparatus
of the flamingos (which would make any other form of food
difficult for the chicks to ingest); and the need for a
convenient food supplement when breeding on the barren
Antarctic ice shelf favored by penguins.